Rieckhoff enlisted in the U.S. Army Reserve on September 15, 1998 and completed Basic Combat Training and Advanced Individual Training at Fort McClellan, Alabama.[2] He then served in the U.S. Army Reserve, as a specialist with the 812th Military Police Company. While working on Wall Street in 1999, Rieckhoff transferred to the New York Army National Guard. He graduated from Officer Candidate School in June 2001 and was named a Distinguished Military Graduate. Rieckhoff selected infantry as his branch and joined A Company, 1-105th Infantry (Light).

Rieckhoff left Wall Street on September 7, 2001 with plans to travel and complete additional military schooling. On the morning of September 11, Rieckhoff was at his apartment in Manhattan when the first plane hit the World Trade Center. He participated in the rescue efforts at ground zero.[3] His unit was formally activated for rescue and security operations later that evening.

Rieckhoff recounted his 9/11 experiences for the 9/11 Tribute Center for a project called "Stories of 9/11 told by those who were there"[4]

In 2002, Rieckhoff volunteered for the invasion of Iraq. In January of that year, he was on a plane to join the 3rd Infantry Division at Fort Stewart, Georgia. Rieckhoff was then assigned as a platoon leader in the 3-124th Infantry (Air Assault) of the Florida Army National Guard. The unit was attached to 1st Brigade, 3rd Infantry Division and spent almost a year conducting combat operations in Baghdad, Iraq. Third Platoon conducted over 1,000 dismounted and mounted combat patrols. All thirty-eight of the men in Rieckhoff's platoon returned home alive.

In 2008, Rieckhoff and IAVA led the passage of the "Post-9/11 (New) GI Bill." Despite surprising opposition, IAVA ensured the bill's passage—which has now successfully supported the education of over one million veterans of Iraq and Afghanistan.[6] On February 7. 2007, Rieckhoff testified before the House Veterans Affairs Committee to advocate for passage of the bill.[7]

On February 13, 2014, under Rieckhoff's leadership, IAVA led the creation, passage and signing into law of the historic Clay Hunt Suicide Prevention for America's Veterans (SAV) Act. The law was named after Marine CPL Clay Hunt, a sniper, IAVA member and personal friend of Rieckhoff and other IAVA leaders. Rieckhoff stood on stage with President Obama, Clay's mother, Susan and other national leaders for a signing ceremony at the White House.[8]

In a talk at Johnson Chapel Aug. 31, Paul Rieckhoff offered first-year and transfer students his take on 50 things to do before graduating at the annual DeMott Lecture. Among them: write, write, beat Williams and learn about President John F. Kennedy's Amherst speech—delivered just days before his assassination.[9][10]

Rieckhoff has been an outspoken leader for veterans who has frequently challenged Presidents, political leaders, and particularly former VA Secretary Eric Shinseki.[11] The Washington Post, in a profile of Rieckhoff's battle with Shinseki wrote, "Younger veterans groups have adopted many of the lessons of today’s fast-moving, hyper-partisan political campaigns to raise the pressure on Shinseki and the VA. Vietnam War veterans are looking for answers about the alleged cover-up of a backlog at a Veterans Affairs hospital in Phoenix. No group exemplifies this shift more clearly than Iraq and Afghanistan Veterans of America, which is based in New York and has about 270,000 members who have signed up for its e-mail list. IAVA’s 45-person staff — most of them younger than 30 — is small compared to the larger, more established veterans organizations. Like the traditional veterans groups, IAVA is nonpartisan. But the group and its founder, Paul Rieckhoff, 39, have been a relentless presence on Capitol Hill, online and in the national media, criticizing Shinseki’s leadership of the department. “IAVA is packed with a bunch of digital natives,” said Phil Carter, who studies veterans’ issues for the Center for a New American Security. “That affects the speed that they can get out a critical message and the uptake that their message gets from the media.”

In 2012, Stars and Stripes wrote an article titled, "IAVA attracts the spotlight – and detractors." [12] "IAVA representatives are frequent cable news guests and regulars at hearings on Capitol Hill, where few if any veterans initiatives are passed without their blessing. They’re advertising stars, thanks to donated public service spots and a partnership with Miller High Life. IAVA events drew crowds at the Super Bowl and this year’s presidential political conventions, among dozens of other high-profile events. In just eight years, IAVA has transformed itself from an upstart veterans organization to a lobbying heavyweight and media favorite. For many Americans not connected to the military, they’ve become the face not just of the current combat generation but of all veterans.That infuriates their critics, who see IAVA as a small, unrepresentative sample of returning war heroes, a veterans group with an uncharacteristic liberal bent and a business model that emphasizes online communities over traditional outreach.They’re too loud. They take too much credit. They’re unwilling to wait for change. They’re too convinced that their unconventional strategies and overly aggressive approach are more helpful than what other advocates — and the Department of Veterans Affairs — are offering."

The military blog This Ain't Hell, which prides itself on criticizing Rieckhoff, published a 2004 Amherst Magazine interview of Rieckhoff which included a photograph of him in uniform erroneously wearing a Bronze Star Medal and a United States Army Special Forces unit patch.[13] According to Stars and Stripes, in response to the allegation that he wore a medal he had not been awarded, Rieckhoff "defended the medal as a paperwork mistake" and explained that he "bought his Bronze Star after being told he had earned the medal, but hasn't worn it since that interview." Concerning the allegation that Rieckhoff had worn a Special Forces unit patch without ever having served in a Special Forces unit, Stars and Stripes noted that Rieckhoff "blames the Special Forces patch on bad timing and enthusiasm.[14]" The article states that "[h]e sewed on the patch days after receiving an assignment to the unit, but pulled it off a few weeks later when that assignment changed."

Rieckhoff was a vocal critic of the film, The Hurt Locker, posting a piece in Newsweek, titled "Veterans: Why 'The Hurt Locker' Isn't Reality"[17] and appearing on PBS Newshour to state his position.[18]

Rieckhoff was a strong supporter of the controversial film, American Sniper, writing a review for Variety stating, “'American Sniper' is the single best work of film about the Iraq War ever made."[19] In 2015, Rieckhoff appeared on Comedy Central's The Nightly Show to defend American Sniper as an effective public awareness tool for veterans causes.[20]

Now, with nearly 300,000 veterans and civilian supporters nationwide, IAVA is the only 21st Century veterans’ empowerment organization (VEO) dedicated to standing with the 2.8 million veterans of Iraq and Afghanistan from their first day home through the rest of their lives. Over the last decade, IAVA has become the leading advocate for its community and served over 800,000 post-9/11 veterans. IAVA has pioneered historic changes, like the Post-9/11 GI Bill, VA funding reforms, the VOW to Hire Heroes Act, and more. IAVA is now leading the charge on combating veteran suicide, improving support for female veterans, ending the VA disability claims backlog, and defending veterans’ education benefits.

Rieckhoff is a nationally-recognized and known authority on the new veteran community and policy. He regularly testifies before Congress on issues facing the veterans' community and writes regularly for national websites and publications. In August 2011, Rieckhoff and four other IAVA Member Veterans appeared on the cover of TIME magazine for a feature about Iraq and Afghanistan veterans being leaders of the New Greatest Generation.[21]

Rieckhoff wrote a book describing his experiences in Iraq and activism afterwards entitled Chasing Ghosts (2006).[1] NPR described it in a review: "The true story of the first soldier to challenge the war in Iraq provides a grunt's-eye view of the battles on the streets of Baghdad, and a patriot's vision of where America has gone wrong and how it can reset its path. As an infantry platoon leader, Rieckhoff was proud to follow in the footsteps of his father and grandfather. He and his soldiers spent almost a year in one of the most dangerous and volatile areas of Baghdad—and what they encountered there was chaos: not enough troops, no humanitarian aid, no body armor, no radios, and no real plan. He saw what happened when we tried to conduct a war on the cheap. What followed, over the next ten months, set him on a course that would forever change his life. When he finally came home, Rieckhoff vowed to tell Americans the truth. —From publisher description. A veteran of the war in Iraq provides an infantry soldier's view of the war and the challenges confronting the American military in the field due to poor planning, lack of accountability and preparedness, inadequate protection, and other problems."[23]

Rieckhoff was inducted into the Global Ashoka Fellowship in 2010 as recognition of his innovation and entrepreneurship on behalf of new veterans.[26] Rieckhoff is also a member of the Council on Foreign Relations.

Rieckhoff is an advocate for Iraqi and Afghan refugees and interpreters, and an advisory board member of The List Project, "a non-profit operating in the U.S., founded with the belief that the United States Government has a clear and urgent moral obligation to resettle to safety Iraqis who are imperiled due to their affiliation with the United States of America." [27] He helped bring his former Iraqi translator, Esam Pacha (who was targeted for assassination, to the US.[28] Pacha is now a thriving artist living in Connecticut.[29]

Named #37 of GQ’s "50 Most Powerful People in D.C.]" in 2009,[30] Rieckhoff has been honored with the Common Ground “Celebrating Home Award” and the Generation Engage “Lewis Cullman Civic Engagement Award” for his leadership in the service community. In 2004, he was also honored by Esquire magazine as one of “America’s Best and Brightest.”

Rolling Stone named Rieckhoff to its list of "The Quiet Ones: 12 Leaders Who Get Things Done" [31] Rolling Stone wrote, "A former first lieutenant who served a tour of duty in Iraq, Rieckhoff launched the nation's largest advocacy group for veterans of Bush's two wars. This fall, he used his lobbying clout to spearhead the near-unanimous passage of the Veterans Opportunity to Work to Hire Heroes Act. The new law addresses the staggeringly high unemployment rate for returning veterans by providing businesses with a tax credit of up to $9,600 for each vet hired. The law also expands funding for education and vocational training for all veterans, including those from past wars. Rieckhoff's advocacy was also essential in securing passage of the New GI Bill under Bush in 2008, and its expansion under Obama in 2010."

Rieckhoff was awarded an honorary Doctor of Humane Letters by Amherst College in 2013.[33]

The Hill named Rieckhoff to its "Top Lobbyists 2014: Grassroots" list for the second year in a row, adding, "The Iraq veterans organization played a leading role on passing a Department of Veterans Affairs reform bill this year, and will continue to be a big player on its implementation, among other veterans issues."[34] In 2013, writing "Rieckhoff champions legislation to help veterans of the Iraq and Afghanistan wars deal with brain trauma and post-traumatic stress disorder."[35]

In 2014, Rieckhoff was awarded a "Teddy Award" by TIME Magazine for his political courage in demanding VA reform throughout years of scandal. "Paul Rieckhoff of Iraq and Afghanistan Veterans of America has been an edgy, controversial figure in recent years, a creative critic of the Department of Veterans Affairs. The revelation of widespread corruption and incompetence in the VA, and the dismissal of Secretary Eric Shinseki, proved the value of Rieckhoff’s persistence. His willingness to take flak for his brothers and sisters in arms merits a Teddy."

In December 2014, Task and Purpose named Rieckhoff to "The Mighty 25: Veterans Poised to Make a Difference in 2015".[36] "Iraq and Afghanistan Veterans of America marked a decade of existence in 2014, and the organization is showing no signs of slowing down going into next year. Under the leadership of the well-networked and media-savvy founder Paul Rieckhoff, IAVA championed the Clay Hunt SAV Act – legislation designed to combat the veteran suicide rate – at the end of the year, although the bill’s passage was singularly impeded by Sen. Tom Coburn."